Nanoparticles are particles with a diameter ranging from about 1 nm to 1000 nm. There are different types of nanoparticles available in the market place. Metals (such as copper, gold, and silver), metal oxides (such as zinc oxide, copper oxide), diamond and other forms of carbon have been used to make their respective nanoparticles. These particles have demonstrated applications in several fields such as drug-delivery, cosmetics formulations and heat transfer fluids.
The important properties for a heat transfer fluid in relation to heat transfer capabilities are the following: specific heat capacity (Cp), thermal conductivity (k), viscosity (μ), and density (ρ). Specific heat capacity (Cp) is a function of the energy (thermal) storage capacity of a fluid and it also affects the fluid side heat transfer coefficient in heat exchangers. Thermal conductivity (k) affects the heat transfer coefficient more significantly than the other parameters. Viscosity and density are also very important for heat transfer as well as pumping power requirement for the heat transfer fluid.
The most commonly used fluids for cooling applications are water and fluorocarbons. An example of a fluorocarbon based heat transfer fluid is FC-77, a 3M Corporation product. Water has excellent thermal properties such as a high latent heat of evaporation, specific heat and thermal conductivity, but is not adequate for single-phase liquid cooling of high heat flux applications. In addition, water cannot be used in applications where ambient temperature may reach below 0° C. Water/glycol mixtures provide a low freezing point, but their thermo-physical properties are inferior compared to pure water. Fluorocarbons are inert and dielectric, and are therefore used in immersion as well as spray cooling applications in direct contact with electronics circuits. However, their specific heat and thermal conductivity are far worse than water and glycol/water mixtures and they are extremely expensive (>$200/gallon). All these fluids do not have an in-built energy storage mechanism other than the sensible heat, i.e., specific heat capacity.
The method and composition disclosed herein is for a hybrid nanoparticle that can be used as a component in a fluid or other media to provide enhanced thermal storage capability, thereby increasing the specific heat capacity as well as the thermal conductivity of the base fluid. The concept is to utilize the heat of fusion of a phase change material, for example paraffin, to absorb thermal energy from the heat source and then release it in a heat sink/radiator during the solidification of the phase change material.
Also disclosed herein is a fluid that utilizes hybrid nanoparticles to increase the specific heat capacity as well as thermal conductivity of the heat transfer fluid. Phase change materials (PCM) in an encapsulated form (about 100 microns) have been tried as a means to increase the specific heat (Cp) of a fluid by utilizing their latent heat of fusion, whereas copper and other nanoparticles have been incorporated into heat transfer fluids to increase the thermal conductivity (k) of the heat transfer fluids. PCMs have been very successful in static/passive thermal management systems for example, thermal interface materials, body suits/vests, cold/thermal storage, but in heat transfer fluid applications there have been problems of mechanical damage, supercooling of the PCMs inside microcapsules, agglomeration, and blockage of the heat exchanger channels.
Nanofluids, i.e., suspensions of metal or metal oxide nanoparticles in a base fluid have been developed to increase the thermal conductivity of the base fluid. However, addition of nanoparticles to a fluid does not increase the specific heat capacity, i.e., the energy storage capacity of the base fluid. Therefore, a nano-fluid may be able to dissipate higher heat fluxes, but it does not have good thermal storage or transport capability.
Heat transfer fluids currently available do not provide a high heat flux and have limited applications where high heat flux is required. The technical challenge is to develop a water-based fluid with low freezing point and very high heat capacity and thermal conductivity.
Disclosed herein is a method and composition for hybrid nanoparticles, and a composition of a heat transfer fluid medium utilizing the hybrid nanoparticles. The hybrid nanoparticle has an outer layer that has very high thermal conductivity and an inner core that comprises a phase change material (PCM) that has thermal storage capability due to the latent heat absorbed or released during phase change. These hybrid nanoparticles can also be used in other process applications, for example, in a heat transfer gel, thermal interface materials, sensors and biomedical applications.